The present invention relates to an optical compensatory sheet comprising a cellulose acetate support and an optically anisotropic layer containing a discotic liquid crystal molecule. The invention also relates to an ellipsoidal polarizing plate comprising a transparent protective film, a polarizing membrane, a cellulose acetate support and an optically anisotropic layer in this order. The invention also relates to a liquid crystal display using the optical compensatory sheet.
A cellulose acetate film is used in various photographic or optical elements because it has tough and enough flame retardant properties. The cellulose acetate film is used in a liquid crystal display (LCD). The cellulose acetate film has a high optical isotropy (a low birefringence) compared with other polymer films. Accordingly, the cellulose acetate film is usually used as an optical element requiring an optical isotropy, such as a protective film of a polarizing element or a color filter.
On the other hand, an optical compensatory sheet requires a high optical anisotropy (a high birefringence). Accordingly, a synthetic polymer film having a high birefringence, such as a polycarbonate film or a polysulfone film is usually used as the optical compensatory sheet.
Japanese Patent Provisional Publication Nos. 3(1991)-9325, 6(1994)-148429, 8(1996)-50206, 9(1997)-26572, U.S. Pat. Nos. 5,583,679 and 5,646,703 disclose another optical compensatory sheet, which comprises a transparent support and an optically anisotropic layer containing a discotic liquid crystal molecule. The high birefringence of the optical compensatory sheet is obtained by the optically anisotropic layer containing the discotic liquid crystal molecule. A cellulose acetate film is usually used as the transparent support, because the support preferably has a high optical isotropy (a low birefringence).
The above-mentioned optical compensatory sheet using the discotic liquid crystal molecule has usually been constructed to optically compensate a liquid crystal cell of a twisted nematic (TN) mode, which is used in a TFT liquid crystal display. If the optical compensatory sheet is used for a liquid crystal cell of another mode, such as a veritably aligned (VA) mode, an optically compensatory bend (OCB) mode or a hybrid aligned nematic (HAN) mode, the sheet cannot completely compensate the cell.
The optically anisotropic layer containing the discotic liquid crystal molecule can be used in combination with an optically anisotropic support. The optical anisotropy of the layer is combined with the optical anisotropy of the support to optically compensate the cell of the VA, OCB or HAN mode. A synthetic polymer film having a high birefringence, such as a polycarbonate film or a polysulfone film can be used as the optically anisotropic support. However, the synthetic polymer film is inferior as a support (physical properties and affinity with a coating layer). Therefore, the optically anisotropic support preferably is a lamination of the synthetic polymer film having a high birefringence and a cellulose acetate film, which is an excellent support, but has a low birefringence.
As is described above, a synthetic polymer film is used where the film should have an optical anisotropy (a high birefringence), while a cellulose acetate film is used where the film should have an optical isotropy (a low birefringence). This is a general principle in the technical field of an optical material such as an optical compensatory sheet.
The present inventors have tried to use a cellulose acetate film where the film should have an optical anisotropy (a high birefringence) against the general principle. The cellulose acetate film is superior as a support compared with a synthetic polymer film. If a cellulose acetate film has a high optical anisotropy (a high birefringence), the cellulose acetate film can be used as an optically anisotropic support of an optical compensatory sheet.
However, a cellulose acetate film having a low birefringence has been considered excellent according to prior art. Therefore, a method of increasing the birefringence of the cellulose acetate film has scarcely been studied, while a method of decreasing the birefringence has intensively been studied.
The present inventors have studied the method of increasing the birefringence of the cellulose acetate film, and have succeeded in obtaining a cellulose acetate film having a high birefringence.
An object of the present invention is to provide an optical compensatory sheet comprising a cellulose acetate film having a high birefringence and an optically anisotropic layer containing a discotic liquid crystal molecule.
Another object of the invention is to provide an ellipsoidal polarizing plate or a liquid crystal display using a cellulose acetate film as an optically anisotropic support.
The present invention provides an optical compensatory sheet comprising a cellulose acetate support comprising cellulose acetate, and an optically anisotropic layer containing a discotic liquid crystal molecule, wherein the cellulose acetate support has a Bth550 birefringence defined by the following formula in the range of 0.0007 to 0.004:
Bth550={(nx+ny)/2}xe2x88x92nz
in which each of nx and ny is a principal refractive index measured by light of 550 nm in plane of the support; and nz is a principal refractive index measured by light of 550 nm along a thickness direction of the support.
The invention also provides an ellipsoidal polarizing plate comprising a transparent protective film, a polarizing membrane, a cellulose acetate support and an optically anisotropic layer containing a discotic liquid crystal molecule in this order, wherein the cellulose acetate support has a Bth550 birefringence defined by the following formula in the range of 0.0007 to 0.004:
Bth550={(nx+ny)/2}xe2x88x92nz
in which each of nx and ny is a principal refractive index measured by light of 550 nm in plane of the support; and nz is a principal refractive index measured by light of 550 nm along a thickness direction of the support.
The invention further provides a liquid crystal display comprising a liquid crystal cell and two polarizing elements arranged on both sides of the liquid crystal cell, at least one of said polarizing elements being an ellipsoidal polarizing plate comprising a transparent protective film, a polarizing membrane, a cellulose acetate support and an optically anisotropic layer containing a discotic liquid crystal molecule in this order, wherein the cellulose acetate support has a Bth550 birefringence defined by the following formula in the range of 0.0007 to 0.004:
Bth550={(nx+ny)/2}xe2x88x92nz
in which each of nx and ny is a principal refractive index measured by light of 550 nm in plane of the support; and nz is a principal refractive index measured by light of 550 nm along a thickness direction of the support.
The present inventors have succeeded on preparation of a cellulose acetate film having a Bth550 birefringence in the range of 0.0007 to 0.004. The film can be prepared by (1) using a birefringence increasing agent, (2) adjusting an acetic acid content of cellulose acetate, or (3) a specific film forming method. The methods (1) to (3) of increasing the birefringence are described below. The obtained cellulose acetate film can be used as an optically anisotropic support of an optical compensatory sheet.
The optical compensatory sheet having the cellulose acetate support can advantageously be used in a liquid crystal display, particularly a display of a vertically aligned (VA), optically compensatory bend (OCB) or hybrid aligned nematic (HAN) mode.